Barrier miking, also known as proximity miking, is the technique of mounting a microphone on or very near an acoustically reflective surface. Mounting microphones on barriers, baffles, acoustic boundaries and other surfaces is old in the art and is used to help eliminate acoustic interference or distortion caused as a result of direct and reflected sound waves from the same source arriving at the microphone at different times.
One approach to barrier miking has been the placement of the microphone very close to the floor to reduce the effects of the reflections from the floor boundary. An article by Roger Anderson and Robert Schulein, "A Distant Miking Technique" dB Magazine, Vol. 5, No. 4, pp 29-31 (April 1971), describes a method in which the diaphragm of the microphone is perpendicular to the floor.
Another barrier miking technique described in U.S. Pat. No. 4,361,736 issued to Edward M. Long and Ronald J. Wickersham places the diaphragm of the microphone in a plane substantially parallel to the boundary surface and a small distance from it.
The additional advantage of boundary miking is that direct sound waves couple, via the boundary, with the reflected waves. If the microphone diaphragm is sufficiently close to the boundary surface, the direct and reflected sound waves stay in approximately the same phase relationship up to the highest audible frequencies.
One application of this technique, described in an article by Michael E. Lamm and John C. Lehmann entitled "Realistic Stereo Miking for Classical Recording," Recording Engineer/Producer (Aug. 1983) mounts the microphones on the surface of large plexiglass sheets which are hung down from the ceiling of the performance auditorium.
For stereo recording, additional techniques have been employed to increase the accuracy of sound localization (the human ability to accurately pinpoint a source of sound) with special microphone arrangements.
Generally, the most accuracy has been obtained by mimicking the actual human head, using molded plastic, rubber or fiberglass constructions which approximate the human head in size and proportions, particularly in construction of ear pinnae and ear channels.
Designs of this sort, such as are marketed by Japan Victor Corporation and Neumann, A.G., place microphones in the molded "ears" or "ear canals" of the dummy head and closely approximate the human hearing perspective. Recordings made by this method are termed "binaural" or "dummy-head stereo."
A more generalized approach, far more suitable for loudspeaker reproduction than binaural, was described by Alan D. Blumlein in British Pat. No. 394,325 (June 14, 1933). Blumlein described several methods for placing two microphones in immediate proximity to one another (since termed "coincident"), and aimed outwardly from a centerline from the source of sound at angles right and left by approximately 45.degree.. This technique is generally described as the "Blumlein" method.
Blumlein also designed microphone circuitry utilizing two bi-directional microphone elements in the same vertical plane, put at right angles to one another (one facing the source of sound and the other at right angles to it). By utilizing sum-and-difference circuitry described in his patent, fairly convincing localization could be achieved for listeners using loudspeaker reproduction, with the additional advantage that the two coincident microphone signals sum accurately for monaural reproduction. This technique is now popularly termed "mid-side."
Other near-coincident microphone techniques using directional cardioid microphones about ear distance apart, evolved from Blumlein, are still popularly utilized for classical musical recordings and are variously termed "X-Y," "NOS," and "ORTF."
Blumlein also describes a means for separating two microphones by baffling material, which has since been utilized extensively by the British Broadcasting Corporation (BBC). This system, disclosed in more detail in an article by Ron Streicher and Wes Dooley entitled "Basic Stereo Microphone Perspectives--A Review," J. Audio Eng. Soc., Vol. 33, No. 7/8 (July/Aug. 1985), shows two omnidirectional microphones placed on either side of a sound absorptive baffle.
Separated from one another by a distance of six to eight inches, the two omni-directional microphones plus the separating barrier approximate some of the characteristics of binaural recording but this arrangement lends itself more satisfactorily toward loudspeaker reproduction than dummy-head binaural.
Each of the two-microphone systems described above has, with the exception of dummy-head stereo, shortcomings which do not result in a close duplication of the hearing characteristics of the human ear or in some way limit the environment for recording.
The limitations of these approaches include, for example, large size and unwieldiness (Lamm and Lehmann); off-axis coloration and uneven pickup field (X-Y, NOS, ORTF); low and mid-frequency induced phase errors and right-left muddying (BBC); lack of time-of-arrival localization cues (Blumlein, Mid-Side) and lack of essential head-shadow localizations cues due to left/right overlap at mid-to-high frequencies (all except BBC).
Binaural dummy-head methods are extremely rich in head-shadow and time-of-arrival cues and add further complex defraction effects due to the introduction of the ear pinnae shape. These complex wave forms enable approximately 40% of headphone listeners to not only accurately localize on the median plane, but to obtain additional height and front-to-back localization as well.
However, the excellence of binaural recording systems in matched with pragmatic limitations. Headphones must be used by the listener for localization to occur, and with headphones on, approximately 60% experience significant inaccuracies of localization (many experiencing sounds from the rear that originally occurred in front) because individual pinnae structures differ significantly from those molded on the idealized dummy head.
When reproduced from speakers, binaural localization is quite poor and muddied by the complex pinnae-created wave forms. The binaural right/left signals do not sum well to mono, and using the dummy head under field conditions is cumbersome, ungainly and (in public settings) very attention-getting.
It is an object of this invention to overcome the limitations presently existing in binaural, coincident and near-coincident stereo recording methods.
It is a further object of this invention to create right and left microphone signals which sum to mono without distortion or phase-shift caused comb filtration.
More specifically, it is an object of the present invention to provide means that mimic the pickup angles of the human ear.
A further object of the present invention is to provide means by which reception of sound, particularly at higher frequencies (over 2500 Hz), is distinctly different in each microphone channel, while at lower frequencies there is a dual pickup of sound by both microphones which includes time delay phasing errors that simulate those heard by the human ear.
A further object of this invention is to produce right and left microphone signals which, when reproduced on loudspeakers, provide extremely accurate localization, and when reproduced with headphones provide some of the vertical and front-to-back localization normally associated with dummy-head binaural recording.
A further object of the present invention is to provide an improved simulated binaural recording system in which boundary plates are used in association with microphones for purposes of reinforcing frequencies in the audible frequency range arising from the microphone side, and to effectively achieve a flat frequency response for all sounds received from above the boundary while substantially attenuating signals from the other side.
A further object of this invention is to combine the right-and-left low frequencies picked up below 700 Hz in such a way as to overcome the low-frequency attenuation usually associated with boundary plates less than two feet in width and depth.
A further object of the invention is to provide a system which provides a more realistic recording than the prior art by closely duplicating the hearing characteristics of a human head, while eliminating most of the complex waveforms formed by defraction around the human pinna.
The invention also provides a system which is very accommodating of extremes of sound pressure, performance dynamics and source distance.
It is a still further object of the invention to provide a small binaural recording system which is readily portable and usable in a variety of contexts where ease of operation, light weight, and relative unobtrusiveness are desirable.